This relates to graphics displays and, more particularly, to displays of computer graphics on raster video terminals.
In recent years, decreasing memory costs have resulted in the increased use of raster scan displays in computer graphics applications. Raster scanning is particularly well suited for presentation of continuous tone and color images, and perhaps for that reason it also is the display mode used by essentially all computer terminals. Raster type terminals, however, have inherent spatial resolution constraints because the image is constructed from spaced horizontal scans and regularly spaced picture elements (pixels) in each scan. Bandwidth limitations and hardware cost considerations drive the art and, therefore, most raster display systems offer the lowest spatial resolution that is acceptable to the target group. To further reduce cost, some graphics systems reduce the intensity resolution by employing a single bit to deine the intensity of each pixel. Use of a single bit per pixel, of course, permits the use of a smaller frame buffer memory. The spatial resolution limitations of raster scans, coupled with single bit per pixel buffers, generally result in poor image qualtiy which manifests itself in staircase-like patterns in lines that are other than perpendicular or horizontal. A number of artisans have been trying to overcome this aliasing phenomenon.
Akira Fujimoto and Kansei Iwata describe one approach in "Jag Free Images on a Raster CRT", Computer Graphics Theory and Applications, Proceedings of Inter Graphics, 1983, pp. 2-15. They compute an intensity for each pixel in proximity to a drawn line based on a Fourier window calculation. The calculations create a line profile having the shape of an equilateral triangle centered on the drawn line. The intensity computed for each pixel is based on the proximity of the pixel to the line and on the slope of the line.
J. T. Whitted, in "Anti-Aliased Line Drawings Using Brush Extrusion", ACM Transactions on Computer Graphics, Vol. 17, July 1983, pp. 151-156, describes a number of other approaches employed in the art. He employs a brush painting approach in combination with super sampling. More specifically, Whitted employs a virtual canvas of 4096 by 4096 pixels and a small pixel array which serves as the brush. The brush is "dragged" across the canvas at the 4096 by 4096 resolution, but the line is displayed at 512 by 512 resolution. Aliasing is avoided by drawing at high resolution and effectively filtering before resampling at the low resolution. Whitted's system works well but requires large amounts of storage.
It is an object of our invention to draw lines on a raster type terminal without creating the aforementioned aliasing phenomenon. It is another object of our invention to provide a method that is simple and fast enough to permit drawing lines in real time. It is still another object of our invention to blend drawn lines in the image background.